In recent years, there has been an increasing demand for thin, lightweight, and fast response display devices. Along with this, research and development for organic EL (Electro Luminescence) displays and FEDs (Field Emission Displays) have been actively conducted. The luminance of an organic EL element included in an organic EL display is substantially proportional to a current flowing through the element and is less susceptible to external factors such as an ambient temperature. Thus, for organic EL displays, it is preferred to apply a current control type drive scheme in which the luminance of an organic EL element is determined by a current value.
Meanwhile, pixel circuits and drive circuits of a display device are made using TFTs (Thin Film Transistors) made of amorphous silicon, low-temperature polycrystal silicon, CG (Continuous Grain) silicon, and the like. A current flowing through a TFT fluctuates depending on the characteristics of the TFT, such as a threshold voltage and mobility, and variations are likely to occur in the threshold voltage and mobility. Hence, it is difficult to make currents flowing through TFTs and an organic EL element match each other between a large number of pixel circuits included in a display. In view of this, a pixel circuit of an organic EL display is provided with a circuit that compensates for variations in the characteristics of a TFT. By the effect of this circuit, variations in the luminance of an organic EL element are suppressed.
Schemes to compensate for variations in the characteristics of a TFT in a current control type drive scheme are broadly divided into a current program scheme in which the amount of current flowing through a driving TFT is controlled by a current signal; and a voltage program scheme in which such an amount of current is controlled by a voltage signal. By using the current program scheme, variations in threshold voltage and mobility can be compensated for, and by using the voltage program scheme, variations in only threshold voltage can be compensated for.
However, the current program scheme has problems. Firstly, since a very small amount of current is handled, it is difficult to design a pixel circuit and a drive circuit. Secondly, since it is susceptible to parasitic capacitance while a current signal is set, it is difficult to achieve an increase in area. On the other hand, in the voltage program scheme, the influence of parasitic capacitance, or the like, is little and a circuit design is relatively simple. In addition, the influence exerted on the amount of current by variations in mobility is smaller than the influence exerted on the amount of current by variations in threshold voltage and the variations in mobility can be suppressed to a certain extent in a TFT fabrication process. Therefore, even a display device to which the voltage program scheme is applied can obtain satisfactory display quality.
For an organic EL display to which a current control type drive scheme is applied, a pixel circuit shown below is conventionally known. FIG. 7 is a circuit diagram of a pixel circuit described in Patent Document 1. A pixel circuit 800 shown in FIG. 7 includes a driving TFT 810, switching TFTs 811 to 814, a capacitor 820, and an organic EL element 830. The switching TFTs 812 and 814 are of an n-channel type and other TFTs are of a p-channel type.
In the pixel circuit 800, the driving TFT 810, the switching TFT 814, and the organic EL element 830 are provided in series between a power supply wiring line Vp and a common cathode Vcom (their potentials are respectively referred to as VDD and VSS). The capacitor 820 and the switching TFT 811 are provided in series between a gate terminal of the driving TFT 810 and a data line Sj. Hereinafter, a connection point between the driving TFT 810 and the capacitor 820 is referred to as A and a connection point between the capacitor 820 and the switching TFT 811 is referred to as B. The switching TFT 812 is provided between the connection point B and the power supply wiring line Vp, and the switching TFT 813 is provided between the connection point A and a drain terminal of the driving TFT 810. All gate terminals of the respective switching TFTs 811 to 814 are connected to a scanning line Gi.
FIG. 8 is a timing chart of the pixel circuit 800. Before time t0, the potential of the scanning line Gi is controlled to a high level. When at time t0 the potential of the scanning line Gi changes to a low level, the switching TFTs 811 and 813 change to a conducting state and the switching TFTs 812 and 814 change to a non-conducting state. The connection point B is thus disconnected from the power supply wiring line Vp and is connected to the data line Sj through the switching TFT 811. In addition, the gate and drain terminals of the driving TFT 810 reach the same potential. Hence, a current flows into the gate terminal of the driving TFT 810 from the power supply wiring line Vp through the driving TFT 810 and the switching TFT 813, and the potential at the connection point A rises while the driving TFT 810 is in a conducting state. The driving TFT 810 changes to a non-conducting state when the gate-source voltage thereof reaches a threshold voltage Vth (negative value) (i.e., the potential at the connection point A reaches (VDD+Vth)). Therefore, the potential at the connection point A rises to (VDD+Vth).
Then, when at time t1 the potential of the data line Sj changes from a previous data potential Vdata0 (a data potential written to a pixel circuit in a previous row) to a present data potential Vdata, the potential at the connection point B changes to Vdata. Therefore, the voltage between electrodes of the capacitor 820 immediately before time t2 is a potential difference (VDD+Vth−Vdata) between the connection point A and the connection point B.
Then, when at time t2 the potential of the scanning line Gi changes to a high level, the switching TFTs 811 and 813 change to a non-conducting state and the switching TFTs 812 and 814 change to a conducting state. The gate terminal of the driving TFT 810 is thus disconnected from the drain terminal thereof. In addition, the connection point B is disconnected from the data line Sj and is connected to the power supply wiring line Vp through the switching TFT 812. Accordingly, the potential at the connection point B changes from Vdata to VDD and correspondingly the potential at the connection point A changes by the same amount (VDD−Vdata; hereinafter, referred to as VB) and thus reaches (VDD+Vth+VB).
After time t2, since the switching TFT 814 is placed in a conducting state, a current flows through the organic EL element 830 from the power supply wiring line Vp through the driving TFT 810 and the switching TFT 814. The amount of current flowing through the driving TFT 810 increases or decreases depending on the gate terminal potential (VDD+Vth+VB), and even when the threshold voltage Vth is different, if the potential difference VB is the same, then the amount of current is the same. Therefore, regardless of the value of the threshold voltage Vth, an amount of current according to the potential Vdata flows through the organic EL element 830 and thus the organic EL element 830 emits light with a luminance according to the data potential Vdata.
As described above, according to the pixel circuit 800, variations in the threshold voltage of the driving TFT 810 can be compensated for and the organic EL element 830 can emit light with a desired luminance. However, the pixel circuit 800 has a problem that the circuit may not operate properly when variations in the threshold voltage of the driving TFT 810 are compensated for.
For example, when almost no current flows through the driving TFT 810 in a previous frame (when black display is performed), the potential VA at the connection point A at time t0 in FIG. 8 is substantially (VDD+Vth) or higher. When the potential at the connection point B changes from VDD to Vdata during a period from time t0 to time t1, the potential at the connection point A also correspondingly changes. However, since, as described above, Vdata>VDD, if the potential at the connection point B rises from VDD to Vdata when the potential at the connection point A is substantially (VDD+Vth) or higher, then the potential at the connection point A becomes higher than (VDD+Vth). Therefore, the driving TFT 810 maintains a state in which almost no current flows therethrough, and thus, is not placed in a conducting state. In this case, variations in the threshold voltage of the driving TFT 810 cannot be compensated for by the above-described method.
A pixel circuit that solves this problem is also devised. FIG. 9 is a circuit diagram of a pixel circuit described in Patent Document 2. A switching TFT 915 for applying an initialization voltage is added to a pixel circuit 900 shown in FIG. 9. A driving TFT 910, switching TFTs 911 to 914, a capacitor 920, and an organic EL element 930 which are included in the pixel circuit 900 respectively correspond to the driving TFT 810, the switching TFTs 811 to 814, the capacitor 820, and the organic EL element 830 which are included in the pixel circuit 800.
The components (except for the switching TFT 915) of the pixel circuit 900 are comparable to their corresponding components of the pixel circuit 800 and the pixel circuit 900 operates in substantially the same manner as the pixel circuit 800. Note that, to make a pixel circuit that operates in the same manner as the pixel circuit 800 including TFTs having different polarities, by using only TFTs having the same polarity, two split scanning lines G1i and G2i are provided in the pixel circuit 900.
In the pixel circuit 900, the switching TFT 915 is provided between an initialization power supply wiring line Vint and a drain terminal of the driving TFT 910, and the switching TFTs 913 and 915 are controlled to a conducting state before starting an operation for compensating for variations in the threshold voltage of the driving TFT 910. In this manner, a potential of the initialization power supply wiring line Vint can be provided to a gate terminal of the driving TFT 910 (connection point A). Hence, by performing an initialization process by providing a potential at which the driving TFT 910 is always placed in a conducting state, to the initialization power supply wiring line Vint, the driving TFT 910 can be set to a conducting state, regardless of a state before initialization. Accordingly, the pixel circuit 900 can operate properly so as to compensate for variations in the threshold voltage of the driving TFT 910, regardless of a previous state thereof.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2005-157308
[Patent Document 2] Japanese Patent Application Laid-Open No. 2007-133369